1. Field of the Invention
This invention relates to a tire pressure monitoring system.
2. Description of the Related Art
An automobile (vehicle) driver conducting an inspection before starting can easily check the engine oil and coolant levels visually from the exterior but has no easy way to ascertain from the exterior whether the pressure of the tires is appropriate.
For overcoming this inconvenience, tire pressure monitoring systems have been proposed in which a sensor unit including a pressure sensor, transmitting antenna and the like is attached to each vehicle tire to transmit an output indicative of the tire pressure, a monitoring unit including receiving antennas is installed at a suitable location on the vehicle for receiving the outputs, and when a detected tire pressure falls below a predetermined pressure, the driver is warned by lighting a warning lamp, as taught in Japanese Laid-open patent Application No. 2000-142043 (in particular, paragraph nos. 0009 to 0013, FIG. 1 and elsewhere).
Methods for detecting tire pressure include, in addition to the above, the well-known method taught by Japanese Laid-open patent publication No. Hei 6 (1993)-92114 (in particular, paragraph no. 0021 and elsewhere) of estimating tire pressure from the output of an ABS (Antilock Brake System) wheel speed sensor.
When the internal temperature of a tire varies, its air pressure changes even though the amount of air in tire remains constant. Specifically, the air pressure rises when the internal temperature of the tire increases and falls when the internal temperature decreases. Because of this, if the tire pressure is adjusted when the internal temperature is higher than the ambient temperature, such as immediately after driving the vehicle, the tire pressure will reach the proper level with pumping of a smaller amount of air than is actually required. Since inflation is therefore discontinued before the actually required amount of air has been pumped or inflated into the tire, the tire pressure will be lower than the proper pressure when the internal temperature falls to the ambient temperature.